In many analytical systems, discovering the nature of an unknown substance normally requires the substance to first be collected. There are detector systems that analyze a fluid flow analyte stream, i.e., vapors or gases, particulates and liquid bound analytes. Some detector systems are based, for example, on an optical analysis that determines analyte characteristics by subjecting a quantity of the analyte to a light beam and measuring the scattering or fluorescence effects. Chromatography detector systems, for example, are sometimes based upon the optical effects produced by analyte samples. Other detector systems utilize chemical analysis, thermal properties, and the like. There are both quantitative and qualitative analysis detector systems.
Before a sample may be analyzed by chromatography or by many other types of analytical techniques, the sample must be collected and then delivered to a detector system. Many samples of interest are available outside of a controlled setting or are present in such low concentrations that special emphasis must be placed on sample collection, with an example being safety testing of environments that humans occupy. There is a recently heightened awareness of the potential for the intentional detonation of explosives or release of chemical or biological agents into environments occupied by humans. The environments might include open or enclosed spaces in work environments, public environments, or military environments, etc. Many building environments with ducted HVAC (heating ventilation and air conditioning) have the potential for the intentional release of TICS or chemical and biological agents into closed or open spaces occupied by military or civilian personnel. Manufacturing operations also have the potential to permit the escape of hazardous chemicals or biological agents into a manufacturing environment or to an external environment surrounding a manufacturing plant.
In some situations, detection may be desirable in a matter of seconds, but in others, an extended period of time may be used for collection before performing an analysis. An example of the latter case involves workers that may be exposed over a time period to unacceptable levels of harmful agents. Another example of the latter case is when cargo containers are transported from country to country by sea, it may be desirable to collect a sample over a period of several days prior to analysis.
In both uncontrolled settings and controlled settings, analytical resolution and the sensitivity of detection is dependent upon the efficiency of analyte collection and the efficacy of delivery of collected analyte to a detection system. It is desirable, for example, to detect very low levels of toxic or hazardous materials in a particular environment. Gas chromatography and other analytical techniques can employ a variety of detector types, and have been demonstrated to be very sensitive types of analysis techniques, for example. Another example is a chemresistor based device, which uses a detector whose resistivity changes when it is exposed to particular chemical vapors. Whatever the type of detector system, however, concentrating analyte in a stage prior to the detector system can improve detection limits for the analyte(s) of interest, and can also provide a more reliable quantitative or qualitative determination of an analyte.
Others have worked on concentrating analytes, and have proposed systems including a micro scale collection device. A group working at Sandia National Laboratory in Albuquerque, N. Mex. has developed chemical preconcentrators including a preconcentrator heated plate that incorporates a sorbent material coating. This work is discussed, for example, in Manginell et al. U.S. Pat. No. 6,257,835, entitled Chemical Preconcentrator with Integral Thermal Flow Sensor and in Manginell et al. U.S. Pat. No. 6,171,378, entitled Chemical Preconcentrator, which are incorporated herein by reference. The chemical preconcentrator used in that work is formed from a substrate having a suspended membrane, such as low-stress silicon nitride. A resistive heating element is deposited over the membrane and coated with a sorbent, such as a hydrophobic sol-gel coating or a polymer coating. A fluid flow is passed over the sorbent to achieve a collection. A high concentration may then be delivered to a detector system by desorbing, which is achieved by heating the resistive heating element.
One advantage of this work by Manginell and others is that it can provide a relatively high concentration of analyte by collecting it over a long period, and then delivering it in a short amount of time. Another advantage is the MEMS (microelectromechanical systems) micro scale of the device and the MEMS fabrication techniques that permit integration of the device with other system components, for example to form a micro analytical system.
In another style of analyte collector, a column that is packed with a porous adsorbent is used to collect analyte by flowing air through the column and thermally desorbing collected material. The pressure drop associated with this sort of device is typically too high for high flow applications and requires higher power consumption. If the amount of adsorbent is minimized to allow higher flows or faster desorption, the dynamic range is compromised.
However, known prior devices have some drawbacks associated with them. With embodiments of the present invention, some or all of these drawbacks are overcome. Some problems in the art relate to the difficulty of determining the quantity of material collected in a collection device prior to delivery to a detector. In order to perform many chemical analysis (and other types) of tests, a minimum and/or an optimal amount of sample is called for. Measurement of the quantity of material available for delivery to a detector is useful for determination the presence of a sufficient or optimal amount of analyte. If a sufficient and/or optimal amount has been collected, further collection is not necessary.
With many prior art collection and preconcentrator systems, determining the amount of analyte collected is difficult or even impossible. A direct measurement of mass can potentially be used in some applications (e.g., compare device mass before and after collection). In many applications, however, the relatively minuscule mass of analyte collected when compared to the mass of the device make this an unattractive and impractical option.
Another unresolved problem in the art relates to the ability to detect non-volatile and other contaminants that accumulate over time on the sorbent. Dust and other non-volatile particulate may contaminate the sorbent over time and begin to lower collection efficiency of the sorbent as its active sites are affected by the contaminants. Determining when contaminants are present and in what quantity, however, is difficult. Because of this difficulty, systems of the prior art are often scheduled for cleaning and removal of contaminants on an arbitrary schedule that risks cleaning the devices too frequently or not frequently enough. Inefficiencies therefore result.